Synthesis of LiNi_0.5Mn_0.5-<i>x</i>Ti<i>_x</i>O₂ by an Emulsion Drying Method and Effect of Ti on Structure and Electrochemical Properties

Abstract

Layered LiNi0.5Mn0.5-xTixO2 was prepared by an emulsion drying method. Solid solution of LiNi0.5Mn0.5-xTixO2 (R3̄m, space group) was formed to x ≤ 0.3, and when x > 0.3, the layered structure transformed to the simple cubic structure. Rietveld refinement of X-ray diffraction data clearly showed that a small amount of Ti doping into LiNi0.5Mn0.5O2 structure resulted in reduced cation mixing in the Li layer, and the stronger Ti−O bond relative to the Mn−O one would stabilize the crystal structure. Consequently, charge−discharge capacity and Li+ chemical diffusion of Li/LiNi0.5Mn0.5-xTixO2 cells were enhanced by the improvement of physical properties in the oxide matrix. For a higher level of Ti doping, the obtained capacity decreased because a large amount of electro-inactive Ti4+ (d0) depressed the conduction of electrons in the oxide. The cyclability of Li/LiNi0.5Mn0.5-xTixO2 (x = 0−0.3) cells was also dependent on the amount of Ti because of a different degree of cation mixing. In situ XRD observation confirmed that the variation in c-axis was different by increasing the Ti doping amount. That is, the Ti doping resulted in a smaller variation in the c-axis, which would be ascribed to the improvement of structural integrity by the stronger bond of Ti−O in the oxide matrix, compared to the Ti-free one. The Ti-doped LiNi0.5Mn0.5-xTixO2 materials also have good thermal safety characteristics at a highly oxidized state, as confirmed by differential scanning calorimetry.